1. Field of the Invention
This invention relates to a process for producing toner particles which constitute a toner for rendering electrostatic latent images visible in image forming processes such as electrophotography, electrostatic recording, magnetic recording and toner jet recording.
2. Description of the Related Art
Conventionally, electrophotography is a process in which fixed images are obtained by forming an electrostatic latent image on a photosensitive member by various means, developing the latent image by the use of a toner to form a toner image, transferring the toner image to a transfer material such as paper as occasion calls, and then fixing the toner image thereto by the action of heat, pressure, heat-and-pressure, or solvent vapor.
In recent years, toners are roughly grouped into a pulverization toner and a toner obtained through granulation by a wet process. The pulverization toner is produced by melt-kneading a colorant into a thermoplastic resin to make the former dispersed uniformly in the latter, thereafter cooling the resultant melt-kneaded product to solidify, finely pulverizing the kneaded product by means of a fine grinding mill, and classifying the resultant finely pulverized product by means of a classifier to obtain toner particles with the desired particle diameter.
Meanwhile, the toner obtained through granulation by a wet process attracts notice because the toner can have small particle diameter, can have sharp particle size distribution, and also is advantageous for incorporating a release agent in a large quantity. As specific processes for producing toners through granulation by a wet process, proposed are processes for producing toners by a suspension polymerization process, an emulsion polymerization process and other various polymerization processes such as a dissolution suspension process making use of, e.g., a polyester or the like obtained separately by polycondensation.
For example, in the suspension polymerization process and the dissolution suspension process, toner particles having the desired particle diameter are formed in a liquid dispersion medium to obtain a toner particle dispersion. Thereafter, the toner particles are separated from the toner particle dispersion by the use of a separation means as typified by a solid-liquid separator such as a filtering equipment, followed by washing to remove impurities. The wet toner particle cakes thus obtained are dried and then optionally classified, and thereafter an additive(s) is/are optionally added to produce a toner (see, e.g., Japanese Patent Application Laid-open No. S51-14895).
In the emulsion polymerization process, first, a monomer composition containing a polymerizable monomer(s), a polymerization initiator and a surface-active agent, and optionally a cross-linking agent, a chain transfer agent and other additives is dispersed in an aqueous medium by the use of a suitable stirrer to carry out polymerization reaction to obtain emulsified resin particles having the desired particle diameter. Thereafter, a colorant is uniformly finely dispersed in an aqueous medium containing a surface-active agent, to make it associate (i.e., agglomerate and fuse) with the emulsified resin particles to obtain a toner particle dispersion having the desired particle diameter. After that, filtration, washing, drying and classification are carried out like those in the suspension polymerization process and the dissolution suspension process (see, e.g., Japanese Patent Application Laid-open No. H05-265252).
The toner particles obtained through granulation by such a wet process are formed in the liquid dispersion medium, and hence their particle surfaces tend to be affected by various components standing dispersed or dissolved in the liquid dispersion medium. For example, in the suspension polymerization, an aqueous medium containing a dispersion stabilizer of various types is commonly used, and this dispersion stabilizer adheres to the surfaces of the toner particles formed.
In the toner formed by the suspension polymerization process, in order to improve its chargeability, a positively charging or negatively charging, charge control agent is incorporated in the polymerizable monomer composition to carry out the polymerization. However, a charge control agent with a high polarity may partly dissolve in the aqueous dispersion medium to adhere to the surfaces of the toner particles formed. Unless various components having adhered to such toner particle surfaces are sufficiently and uniformly washed and removed in the step of filtration and washing after the polymerization, the toner may have a broad charge quantity distribution to tend to cause a decrease in image density and cause fog, especially under conditions of high temperature and high humidity.
Further, where the toner particles are formed by an emulsification agglomeration process, a surface-active agent must be used as an emulsifying agent in a large quantity. If this surface-active agent remains in a large quantity on the surfaces of the toner particles formed, the toner tends to cause the decrease in image density and the fog more remarkably than the toner produced by the suspension polymerization process.
Accordingly, in the process for producing toner particles through granulation by a wet process (hereinafter often “wet-process granulation process), methods for washing the toner particles having been formed are proposed in variety.
For example, a method is proposed in which, using as a filtering washer a belt fitter having a filter cloth and a vacuum tray which are kept in close contact with each other, toner particles are separated from a toner particle dispersion and then washed (see e.g., Japanese Patent Application Laid-open No. 2002-365839). According to this method, the toner particles can be separated from the toner particle dispersion in a good efficiency and then washed to obtain a toner having superior image characteristics.
This method can be said to be a superior separation and washing method. However, in recent years, as user's needs have become diversified, electrophotographic images are demanded to be highly minute images like photographic images. Under such circumstances, there has still been room for improvement.
As one of effective means for obtaining highly minute images in electrophotographic images, it is to make developer toner particles have small particle diameter. Making the toner particles have such small particle diameter by the pulverization process is not preferable because a great energy is required for pulverization. On the other hand, in the wet-process granulation process, it is easy to make toner particles have small particle diameter. However, in making toner particles have small particle diameter, the water may poorly be drawn off (poor water draw-off) when the toner particles are separated from the toner particle dispersion, tending to make the resultant wet toner particles cakes have a high water content. This is considered due to an increase in particle surface area per unit volume of the cakes formed of wet toner particles. This poor water draw-off leads to insufficient wash-away of the above various components having adhered to the toner particle surfaces.
As a method for avoiding such poor water draw-off, a filtering and washing method that utilizes the dilatancy effect is proposed (see, e.g., Japanese Patent Application Laid-open No. 2004-302099). It is described that, in this method, impact and vibration bring the dilatancy effect, which liquefies cakes to achieve a low water content. However, as a result of studies made by the present applicant, this method has been found to be unable to achieve a sufficiently low water content.
The slurry formed upon completion of the polymerization reaction tends to cause bubbles in a large quantity as a result of stirring and so forth. Accordingly, it is studied that the bubbles of the particle-containing slurry are kept decreased before the particles are filtered and washed. If a slurry containing such bubbles is fed to the step of filtration, a problem may arise such that non-uniform cakes are formed to cause faulty dehydration and faulty washing. Where a continuous belt filter, a siphon pillar type centrifuge, a decanter type centrifugal separator or the like is used, it has also been a problem that its use brings about a very low rate or speed at which cakes are formed and washing water is removed.
As a means for keeping bubbles from forming, it is proposed to control stirring conditions (see, e.g., Japanese Patent Application Laid-open No. 2002-214836). From the viewpoint of making gases less mixed up in liquids, this method is effective in keeping bubbles from forming. However, where, e.g., an acid and an alkali are added to the slurry, the acid and alkali added may react with substances dissolved in the liquid phase, to produce gases. As to such bubbles having formed in the interior of the slurry as a result of such chemical reaction, it has been difficult to keep them from forming, if the stirring conditions are merely controlled.
Thus, it has been sought to provide a production process by which bubbles can be kept from forming and can be removed and a slurry little containing bubbles can be fed to the step of filtration and washing; the bubbles being firstly those caused by foaming depending on production process conditions and by foaming due to chemical reaction, and besides those caused by every situation.
Not intended for defoaming, it is also proposed to use a defoaming machine for the purposes of shape control and desolvation (see, e.g., Japanese Patent Application Laid-open No. 2005-10723).